The invention relates to a hydraulic system for utility vehicles, in particular agricultural tractors, for supplying primary and/or auxiliary pressure medium consumers with pressure medium, comprising a pump sucking from a pressure medium tank, said pump being controlled as a function of the load pressure of the pressure medium consumers and supplying a pump pressure exceeding the load pressure by a predetermined control pressure differential, whereby in order to produce a first control pressure differential for operating a primary pressure medium consumer its load pressure acts upon the pressure and flow controller of the pump and in order to produce a second, higher control pressure differential for operating auxiliary pressure medium consumers a pressure exceeding their load pressure is produced by means of an amplifier device, which acts upon the pressure and flow controller.
European Patent EP 110 70 852 A2 describes such hydraulic system with a fixed displacement pump. Assigned to that pump is a device consisting of a pressure control valve with an inlet for an actuating pressure that enables the pump to deliver pressure medium to the pressure medium consumers at a necessary pressure and (flow) output. In the case of this system, for operating both the vehicle external (hereafter: primary and auxiliary) pressure medium consumers, the actuating pressure for the pressure control valve of the pump is picked up between the two orifices of the amplifier device. In order to provide different control pressures as they are needed to produce the various control pressure differentials for these pressure medium consumers, the line containing the orifices is blocked off by means of an additional pressure regulator, whenever a primary pressure medium consumer is in operation and open whenever an auxiliary pressure medium consumer is in operation. A disadvantage here is that the load pressure of the primary pressure medium consumers, which is utilized as actuating pressure for operating said pressure medium consumers is subject to restriction when passing through the one orifice. As a consequence the actuating pressure takes longer to build up and finally the system dynamics are lower as a result.
A further disadvantage of the prior art hydraulic system is apparent if no implement is mounted on the vehicle, i.e. no auxiliary pressure medium consumer is connected to the hydraulic system of the vehicle. In this case it is possible that due to thermal expansion of the pressure medium inside the load pressure line for auxiliary pressure medium consumers or due to a leakage in the pressure regulator adjacent to the orifices, pressure medium flows to the pressure control valve of the pump. The effect of this is automatic restriction of the pump even as far as actuation of the assigned pressure relief valve (pump short-circuit).
The object of the invention is seen as providing a hydraulic system of the type mentioned at the beginning, wherein the disadvantages described are eliminated and which in particular without any time delay makes available the load pressure of a primary pressure medium consumer as actuating pressure for the device assigned to the pump.